The present invention relates to sampling rate conversion, and more particularly to sampling rate conversion using polyphase filters with interpolation to reduce the memory requirements for conversions having a large number of subphases.
In many applications it is desirable to convert a digitized signal from one sampling rate to another according to the formats of the signal required in different devices. The use of polyphase filters to calculate data values for the signal at times other than originally sampled requires a single filter for each desired sample subphase. For example if the ratio of the desired output sampling rate to the input sampling rate is expressed as a ratio of small integers, such as 33/35, there are 33 (or 35) subphases between one rate relative to the other. Each subphase requires one set of filter coefficients stored in memory, or 33 total sets. For sampling rate converter applications, such as converting video standard D2 PAL to D1 PAL, the required number of subphases for accurate conversion is over 709,000! This presents a significant, if not impossible, challenge using the converter model described above, i.e., over 709,000 filters are required with an equal number of sets of filter coefficients.
What is desired is a sampling rate converter using polyphase filters that provides accurate conversion where the ratio between sampling rates is expressed as a ratio of large integers without requiring a filter for each subphase.